Pål AndreassenTore M. Undeland NORPIE 2004 Power device selection and a comparative study of transistor technologies for a Zero Voltage Switching Buck Converter Abstract— The new ultra thin wafer technology makes it possible to make Non Punch Through (NPT) IGBTs in the 600V range, with a lightly doped collector. This new IGBT technology is tested and compared with other competitive transistor technologies. The case study of the Zero Voltage Switching (ZVS) Buck Converter is performed because this topology until now has favored the use of MOSFETs. Introduction-The ZVS QSW Buck Converter The specific ZVS topology examined is known as the Quasi Square Wave (QSW) Converter. Two of the advantages of this topology are Zero Voltage Switching for both transistors and bidirectional power flow. The disadvantages are high transistor peak current and high input/output current ripple. High turn-off current of the main switch tend to increase the turn off losses, especially when minority-carrier devices such as IGBTs are used. Therefore, this topology has not been regarded desirable with IGBTs, at least with older generations of IGBTs. Fig 1. QSW converter topology and switching waveforms Measurement and calculation of transistor turn-off losses The transistors subjected to comparison are two Punch-Through (PT) IGBTs, IRG4BC30KD and FGB30N6S2D, two Non Punch Through (NPT) IGBTs, SGB15N60 and SKB15N60HS, and the CoolMOS SPB20N60S5. The SKB15N60HS is the one optimized for high speed switching. a) IRG4BC30KD b) SKB15N60HS Fig 2. Turn-off transient hard switching It is interesting to see from the turn-off transient of the high speed IGBT that, due to stray capacitance, C ce, and low output inductance the turn-off is capacitive. Fig 3. Hard switching turn off losses as a function of turn-off current Since the turn-off current of the ZVS QSW buck converter will be at least 2 times the current of a hard switched converter, the rate of increase in the turn-off losses with an increase of turn-off current is of interest. The rate of increase in turn-off losses in the SKB15N60HS is less than for the others, hence promising for use in the ZVS QSW converter. When using an external capacitor, the dv/dt is reduced and the current is almost zero when the voltage starts to rise. The tail-bump of the PT IGBT is larger in the beginning and decreases faster compared to the High Speed NPT IGBT. The tail-bump of the SKB15N60HS is low, and decreases very slowly. a) IRG4BC30KD b) SKB15N60HS Fig 4. Turn-off-transient soft switching The turn-off losses are measured for different values of the external capacitor. It shows that the turn-off losses of the SKB15N60HS and the SPB20N60S5 is low even without the use of an external capacitor, and is not reduced much when using an external capacitor. Fig 5. Capacitive turn-off losses at Ioff=16A as a function of external Cr. Calculation of Transistor losses at operating point For comparative study the losses in the transistor Q1, referred to Fig. 1, is calculated at the operating point,Vin= 300V, Vo=150V,Iout= 8A, fs,max= 160 kHz.The ideal option with regard to the conduction losses would be to keep the Cr as low as possible. With regard to this we compare the transistor losses with the internal stray capacitance as resonant capacitor, Cr. Fig 6. Q1 transistor losses at the operating point Conclusion For the specific operating point the CoolMOS has the lowest loss. However, the new high speed NPT technology looks promising for use in the ZVS QSW converter due to the low increase in turn-off losses with increasing turn-off current.